1. Field of the Invention
The present invention relates to a control apparatus for a continuously variable transmission and more particularly to a control apparatus for a belt type continuously variable transmission having a hydraulic pump driven by an engine.
2. Discussion of Prior Arts
A belt-type continuously variable transmission used for an automobile power transmission apparatus includes a primary shaft on a drive side, a primary pulley provided on the primary shaft and of which width of a groove of the pulley is variable, a secondary shaft on a driven side, a secondary pulley provided on the secondary shaft and of which width of a groove of the pulley is variable, and a drive belt looped over these two pulleys. Engine speed is continuously changed by changing a ratio of winding diameters of the drive belt on the respective pulleys and is transmitted to the secondary shaft.
Hydraulic pressure is supplied from a hydraulic pump driven by an engine to respective pulleys and the groove width of the respective pulleys is changed by controlling the hydraulic pressure. The secondary pulley is subjected to a hydraulic pressure supplied from the hydraulic pump, namely a line pressure and the groove width of the secondary pulley is changed by regulating the line pressure with a line pressure control valve. Further, the primary pulley is subjected to a shift control pressure regulated by a shift control pressure regulating valve and the groove width of the primary pulley is changed by regulating this shift control pressure. When the groove width of the respective pulleys is changed, the ratio of winding diameters of the drive belt on the respective pulleys is changed and the engine speed is continuously shifted and is transmitted to the secondary shaft.
A duty-solenoid valve or a linear solenoid valve are used for the shift control pressure regulating valve and the line pressure control valve. The duty-solenoid valve is a valve which is subjected to a so-called duty-control by command signals from a control unit and the linear solenoid valve is a valve of which valve opening ratio is regulated in accordance with an electric current value supplied from a control unit. As disclosed in Japanese Patent Application Laid-open No. Toku-Kai-Hei 11-82725, these solenoid operated valves are established in such a manner that the shift control pressure and the line pressure have a peak value respectively, when a current value supplied to these valves is zero. This is for holding the tension of the drive belt to prevent the transmission from being damaged when a power supply is shut off during running.
However, in thus constituted continuously variable transmission, when an operator of a vehicle turns an ignition switch of an engine off to stop the engine, the operator sets an electric power to be supplied to the respective solenoid valves to zero or set a power source of the control apparatus itself of the continuously variable transmission to a selfshut condition for the purpose of saving power. Accordingly, when the operator of the vehicle turns the ignition switch off to stop the engine, since the respective solenoid valves hold the shift control pressure and the line pressure at a maximum value respectively, the load of the hydraulic pump abruptly increases. As a result, the frequency of chances of the engine stopping in an overlap position where both of the intake and exhaust valves are held open increases. If the engine stops in an overlap position, exhaust gas in the exhaust manifold enters the intake manifold through the intake and exhaust valves in an open condition and as a result the restartability of the engine is exacerbated.
It is an object of the present invention to provide a control apparatus of a continuously variable transmission having no adverse effect on the restartability of an engine of a vehicle. In order to achieve the object, the control apparatus includes an engine switch member for controlling a start and a stop of the engine, a vehicle speed detecting means for detecting a vehicle speed of the vehicle, an engine speed detecting means for detecting an engine speed of, a line pressure reduction mode establishing means for reducing the line pressure, a line pressure reduction mode establishing means for establishing the line pressure to the line pressure reduction mode when the vehicle speed detected by the vehicle speed detecting means is smaller than a first specified value and when the engine speed detected by the engine speed detecting means is smaller than a second specified value and when the engine switch member is turned off to stop the engine.